STAR (In Situ) Pilot Test to Treat Coal Tar LNAPL and DNAPL in a Fine Sand Aquifer

Former Manufactured Gas Plant in Michigan

The subject site is a former Manufactured Gas Plant (MGP) in Michigan. Investigations have shown that the subsurface geology consists of medium-grained sands to a depth of approximately 35 feet below ground surface (ft bgs) with a reddish brown silty clay beneath. The sand unit is impacted with both Light Non Aqueous Phase Liquids (LNAPL) and Dense Non Aqueous Phase Liquids (DNAPL). LNAPL is encountered in a smear zone located near the groundwater table (15 to 24 ft bgs) and typically extends one to three feet above and below. DNAPL is generally encountered on top of the clay unit up to a maximum thickness of eight feet.

in situ star
Image 1: PDE test area showing the four ignition/air injection wells and associated thermocouple network.

Pre-Design Evaluation (PDE)

A STAR Pre-Design Evaluation (PDE) involving a four-well ignition/air injection system targeting both the LNAPL and DNAPL zones (two wells per zone) was conducted to evaluate key full-scale design parameters such Radius of Influence (ROI), combustion front propagation rate, and volatile mass loading.

Self-sustaining smoldering combustion was achieved in both the LNAPL and DNAPL zones. Combustion was maintained in a self-sustaining manner until the ROI was reached and the objectives of the test were met.

in situ star
Image 2: Combustion gas concentrations in the collected vapors (CO and CO2) as a function of time These data are used to calculate mass destruction rates during the process. As seen here, gas generation follows the typical pattern for STAR with a series of peaks/plateus (7 total) as the combustion front propagates to its maximum extent (ROI).

The coal tar mass destruction rate averaged 46 kilograms per day over the 20 day test period (including down time between ignition events) for a total mass of coal tar destroyed of 933 kilograms. The ROI was determined to be approximately 8 feet and the combustion reaction propagated at a rate of approximately one foot per day.

'Before' and 'After' soil cores show the degree of treatment with Total Petroleum Hydrocarbon (TPH) concentrations decreasing from pre-PDE levels on the order of tens of thousands of milligrams per kilogram to post-PDE levels of only a few hundred milligrams per kilogram.

The fraction of contaminant mass volatilized (as opposed to destroyed via combustion) that was subsequently captured and treated was approximately 1%.

in situ star
Image 3: Recorded subsurface temperatures and extent of treatment within the LNAPL zone. Note that propagation to the northwest (upper right in the image) was limited due to treatment (i.e., removal of coal tar) as part of a prior combustion event in this area.

Before and After Photographs of Soil from the Combustion Zone

in situ star
Image 4: Pre-PDE soil core collected at the ignition/air injection well showing the coal tar-impacted soils (TPH concentration = 11,318 mg/kg).
in situ star
Image 5: Pre-PDE soil core collected 8.5 feet southwest of the ignition/air injection well showing the coal tar-impacted soils (TPH concentration = 8,624 mg/kg).
in situ star
Image 6: Post-PDE soil core collected three feet west of the ignition/air injection well showing soils recovered from the treatment zone (TPH concentration = 35 mg/kg).
in situ star
Image 7: Post-PDE soil core collected 7 feet west of the ignition/air injection well showing soils recovered from the treatment zone (TPH concentration = 340 mg/kg).

Conclusions

The STAR technology is a rapid, safe, and low cost remedial alternative for source areas.
The STAR PDE at the Michigan former MGP Site:

  • Demonstrated self-sustaining smoldering combustion (i.e., no energy input into the system following ignition);
  • Targeted and treated both the DNAPL and LNAPL zones;
  • Showed an ROI of approximately 8 feet with a combustion front propagation rate of approximately one foot per day; and
  • Resulted in TPH concentration reductions of two to three orders of magnitude in the treatment zone.

More Information

Download the PDF version

Back to top